Method for preparing a ceramic thermal barrier coating layer having excellent adhesion and thermal durability

ABSTRACT

The present disclosure relates to a method for preparing a ceramic thermal barrier coating layer including (a) preparing a first suspension in which first oxide particles for thermal barrier coating are dispersed and a second suspension in which second oxide particles for thermal barrier coating are dispersed, respectively; (b) forming a first coating layer on a base material by suspension plasma spraying (SPS) using the first suspension; (c) forming a buffer layer on the first coating layer by the suspension plasma spraying using a mixed suspension of the first suspension and the second suspension; and (d) forming a second coating layer on the buffer layer by the suspension plasma spraying using the second suspension.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2017-0161077 filed in the Korean Intellectual Property Office on Nov. 28, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a method for preparing a ceramic thermal barrier coating layer deposited on a surface of a high-temperature superalloy component of a gas turbine engine.

BACKGROUND

Thermal barrier coatings (TBCs) are heat-resistant ceramic coatings deposited on a surface of a high-temperature superalloy component of a gas turbine engine for power or aviation. It enhances the thermal efficiency of the gas turbine by increasing a turbine inlet temperature (TIT).

Yttria-stabilized zirconia (YSZ), widely used in industry, is formed in a metastable tetragonal prime phase (t′-phase) with no phase change depending on a temperature when it is prepared by a plasma spraying technique or an electron beam physical deposition technique as a thermal barrier coating. This t′-phase YSZ is separated into a thermodynamically stable tetragonal prime phase and a cubic phase when exposed to a temperature equal to or above 1200° C. Also, in this t′-phase YSZ, during a cooling process, the tetragonal prime phase is transformed into a monoclinic phase having a large unit volume, and deterioration of a coating layer occurs. Therefore, this t′-phase YSZ has a limited application temperature.

Lanthanide rare-earth zirconates are the next generation thermal barrier coating material that has recently been studied for application to high-efficiency gas turbine engines operating at or above an application temperature of conventional YSZ thermal barrier coatings. Conventional yttria stabilized zirconia has a low thermal conductivity and a relatively high coefficient of thermal expansion as a ceramic, and has high fracture toughness and high-temperature durability of a thermal barrier coating. However, an application temperature thereof is limited to 1200° C. Whereas, the lanthanide rare-earth zirconates have low thermal conductivity compared to the YSZ and has phase stability existing in the cubic phase up to a melting point. However, it is known that the lanthanide rare-earth zirconates have poor adhesion and high-temperature durability when it is prepared into the thermal barrier coating due to its relatively low thermal expansion properties and fracture toughness.

Therefore, the need is increasing that solve the respective problems caused by the formation of the thermal barrier coating with the conventional YSZ or lanthanide rare-earth zirconates and improve the adhesion of the thermal barrier coating layer as well as the high-temperature durability during a thermal cycle.

RELATED ART DOCUMENT

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SUMMARY

It is an object of the present disclosure to provide a method for preparing a two-layer ceramic thermal barrier coating layer. The method may improve adhesion properties of the thermal barrier coating layer by forming the two-layer ceramic thermal barrier coating layer by suspension plasma spraying with a connected suspension supplier suitable for forming an inclined functional coating layer using a suspension in which pre-treated oxide particles are dispersed. The method may also improve high-temperature durability during a thermal cycle by adjusting a thermal expansion coefficient in a vertical direction of coating.

To achieve the foregoing object, the present disclosure proposes a method for preparing a ceramic thermal barrier coating layer including (a) preparing a first suspension in which first oxide particles for thermal barrier coating are dispersed and a second suspension in which second oxide particles for thermal barrier coating are dispersed, respectively; (b) forming a first coating layer on a base material by suspension plasma spraying (SPS) using the first suspension; (c) forming a buffer layer on the first coating layer by the suspension plasma spraying using a mixed suspension of the first suspension and the second suspension; and (d) forming a second coating layer on the buffer layer by the suspension plasma spraying using the second suspension.

Further, the method for preparing a ceramic thermal barrier coating layer proposes that a first oxide for thermal barrier coating is YSZ (Yttria-stabilized zirconia) and a second oxide for thermal barrier coating is rare-earth zirconates.

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the rare-earth zirconates are La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x is 0.24 to 1.65).

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the step (a) includes (a-1) mechanically pulverizing an oxide powder for thermal barrier coating; and (a-2) forming a slurry containing the pulverized oxide powder.

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the step (a) includes (a-1) mixing two or more oxide powders to perform mechanical alloying; and (a-2) forming a slurry containing an oxide powder for thermal spraying coating obtained in the step (a-1).

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the two or more oxide powders include (i) one or more selected from Y₂O₃, Gd₂O³, and La₂O₃, and (ii) one or more selected from ZrO₂ and CeO₂.

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the mechanical pulverizing or mechanical alloying of the oxide powder in the step (a-1) is performed by planetary ball milling, attrition milling, or shaker milling.

Also, the method for preparing a ceramic thermal barrier coating layer further proposes calcining the oxide powder obtained in the step (a-1) prior to the step (a-2).

Further, the method for preparing a ceramic thermal barrier coating layer proposes that in the step (c), the buffer layer is formed so that a content ratio of the first oxide and the second oxide continuously changes in a thickness direction of a coating layer.

Further, the method for preparing a ceramic thermal barrier coating layer proposes that the step (c) is carried out using a suspension plasma spraying apparatus with a suspension supplier, in which the suspension supplier includes a first suspension storage tank in which the first suspension is stored; a second suspension storage tank in which the second suspension is stored; a first transfer pipe for supplying the first suspension from the first suspension storage tank to a plasma spraying gun; a first opening/closing valve provided in the first transfer pipe for controlling a supply amount of the first suspension to the plasma spraying gun; a second transfer pipe for connecting between the first suspension storage tank and the second suspension storage tank and for transferring the second suspension to the first suspension storage tank; a second opening/closing valve provided in the second transfer pipe for controlling a flow rate of the second suspension to the first suspension storage tank; a third transfer pipe for supplying the second suspension from the second suspension storage tank to the plasma spraying gun; and a third opening/closing valve provided in the third transfer pipe for controlling a supply amount of the second suspension to the plasma spraying gun.

According to the method for preparing a two-layer ceramic thermal barrier coating layer using suspension plasma spraying with a connected suspension supplier suitable for forming an inclined functional coating layer according to the present disclosure, a two-layer ceramic thermal barrier coating layer is formed by suspension plasma spraying with a suspension supplier connected using two types of suspensions of YSZ and rare-earth zirconates containing pulverized oxide particles through high energy milling or alloyed composite oxide particles. Here, the rare-earth zirconates are deposited on an upper layer which is a high-temperature portion of a corresponding coating layer, in which the rare-earth zirconates have a pyrochlore or fluorite or a mixed phase of two phases with low thermal conductivity and excellent phase stability at a high temperature, and there is no m-ZrO₂ causing deterioration of the thermal barrier coating. Here, YSZ is deposited on a lower layer which is a low-temperature portion of the corresponding coating layer, in which the YSZ has a thermal expansion coefficient close to that of a substrate and has excellent high-temperature mechanical properties. Accordingly, it is possible to form a two-layer ceramic thermal barrier coating layer which can be used at higher temperatures as compared with conventional YSZ monolayer coatings, and has improved adhesion and high-temperature durability over rare-earth zirconates monolayer coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a suspension supplier for a suspension plasma spraying apparatus according to an embodiment of the present disclosure.

FIG. 2A shows the result of X-ray diffraction (XRD) pattern analysis of La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) powder after the synthesis used in a suspension of a second suspension storage tank in Examples 1 to 4 of the present application.

FIG. 2B is the result of X-ray diffraction (XRD) pattern analysis of a YSZ powder used in a suspension of a first suspension storage tank in Examples 1 to 4 and Comparative Example of the present application.

FIG. 3 is the result of X-ray diffraction (XRD) pattern analysis of a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and a specimen of a monolayer YSZ thermal barrier coating prepared in Comparative Example of the present application.

FIG. 4A is an image of a scanning electron microscope (SEM) showing a cross-sectional microstructure and coating thickness of a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 of the present application.

FIG. 4B is an image of a scanning electron microscope (SEM) showing a cross-sectional microstructure and coating thickness of a specimen of a monolayer YSZ thermal barrier coating prepared in Comparative Example of the present application.

FIG. 5 is an image of a scanning electron microscope (SEM) showing a cross-sectional microstructure of a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 of the present application and the result of EDS line scanning showing the distribution of La, Gd, Zr, and Y elements.

FIG. 6A is images of a coating side and a substrate side after a adhesion test for a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and a specimen of a monolayer YSZ thermal barrier coating prepared in Comparative Example, in which the white color indicates that dropouts have occurred on the coating side and the black color indicates that dropouts have occurred on the substrate side.

FIG. 6B is the result of an adhesion test of a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and a specimen of a monolayer YSZ thermal barrier coating prepared in Comparative Example.

FIG. 7 is the result of an isothermal deterioration test at 1275° C. for a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 of the present application and a specimen of a monolayer YSZ thermal barrier coating prepared in Comparative Example, and it shows the number of cycles until a coating is soundly attached to a substrate.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the present disclosure, a detailed description of related known functions and configurations will be omitted if it is determined that the gist of the present disclosure may be unnecessarily blurred.

Embodiments according to the concept of the present disclosure may be variously modified and may take various forms. Therefore, it is intended to illustrate specific embodiments in the drawings and describe them in detail in this specification or application. However, it is not intended to limit the embodiments according to the concepts of the present disclosure to particular forms of disclosure. It is to be understood that the present disclosure covers all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.

Terms used herein are only used to describe particular embodiments, but are not intended to limit the present disclosure. Unless the context clearly means otherwise, singular expressions include plural expressions. It should be understood that, herein, the terms “comprises” or “have,” etc. are intended to specify that there is a stated feature, number, step, operation, component, part, or a combination thereof, but it does not exclude in advance the possibility of presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

Hereinafter, the present disclosure will be described in detail.

A method for preparing a ceramic thermal barrier coating layer having excellent adhesion and thermal durability according to an embodiment of the present disclosure includes (a) preparing a first suspension in which first oxide particles for thermal barrier coating are dispersed and a second suspension in which second oxide particles for thermal barrier coating are dispersed, respectively; (b) forming a first coating layer on a base material by suspension plasma spraying (SPS) using the first suspension; (c) forming a buffer layer on the first coating layer by the suspension plasma spraying using a mixed suspension of the first suspension and the second suspension; and (d) forming a second coating layer on the buffer layer by the suspension plasma spraying using the second suspension. Hereinafter, each step will be described in detail.

A first oxide for the thermal barrier coating may be an oxide which has a low thermal conductivity, a relatively high coefficient of thermal expansion as a ceramic, and excellent high-temperature durability of a thermal barrier coating due to high fracture toughness, such as YSZ (Yttria-stabilized zirconia). A second oxide for the thermal barrier coating may be an oxide which has low thermal conductivity and excellent phase stability to a melting point, such as rare-earth zirconates. An example of the rare-earth zirconates may include La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x is 0.24 to 1.65), but are not limited thereto.

In the step (a), a suspension in which oxide particles are dispersed is formed as a feedstock used for the suspension plasma spraying. In the step (a), an oxide fine powder for the thermal barrier coating is obtained through high-energy milling from one or two or more oxide powders as a raw material for forming the thermal barrier coating layer, and then the oxide fine powder is dispersed in a solvent to prepare the first suspension and the second suspension separately.

For example, when one oxide powder such as commercial YSZ (Yttria-stabilized zirconia) is used as a raw material, the suspension is formed through (a-1) mechanically pulverizing an oxide powder for the thermal barrier coating; and (a-2) forming a slurry containing the pulverized oxide powder.

A mechanical milling method capable of applying high energy may be used in the step (a-1). Specific means for this purpose may include planetary ball milling, attrition milling, or shaker milling.

In the step (a-2), the oxide powder in a pulverized size is homogeneously dispersed in a solvent such as ethanol through a means such as ball milling.

In carrying out the step (a), when the oxide for the thermal barrier coating such as rare-earth zirconates is prepared by using two or more commercially available oxide powders as a raw material, the suspension is formed through (a-1) mixing two or more oxide powders to perform mechanical pulverization or mechanical alloying; and (a-2) forming a slurry containing an oxide powder obtained in the step (a-1).

In the step (a-1), the two or more oxides may include (i) one or more selected from Y₂O₃, Gd₂O₃, and La₂O₃, and (ii) one or more selected from ZrO₂ and CeO₂. The two or more oxide powders are mixed and then subjected to mechanical pulverization or mechanical alloying through high energy milling such as planetary ball milling, attrition milling, or shaker milling.

In the step (a-2), a composite oxide powder through the previous step is homogeneously dispersed in a solvent such as ethanol through a means such as ball milling.

In performing the step (a), after performing the pulverization or mechanical alloying of the oxide particles in the step (a-1) and before carrying out (a-2), calcining may be additionally performed in an oxidizing atmosphere if necessary in order to degrade impurities such as organic substances or to synthesize a final oxide. Here, the calcination may be performed at a temperature and for a time sufficient for a compound to be synthesized while the impurities are completely removed.

Next, in the step (b), a first coating layer is formed on the base material by the suspension plasma spraying (SPS) using the first suspension.

Here, the suspension plasma spraying is a spraying method in which liquid suspensions are fed directly into a plasma jet instead of a powder material, in which the suspension injected into the plasma jet is atomized in the plasma jet, and a coating layer is formed through a series of processes such as evaporation of a solvent by heating, dissolution of a material, and collision in a base material.

Next, in the step (c), a buffer layer is formed on the first coating layer by the suspension plasma spraying using a mixed suspension of the first suspension and the second suspension. The buffer layer may be formed such that a content ratio of the first oxide and the second oxide in a thickness direction continuously changes. More specifically, a buffer layer may be formed such that a fraction of the first oxide continuously decreases while a fraction of the second oxide continuously increases from a lower end toward an upper end of the buffer layer.

Finally, in the step (d), a second coating layer is formed on the buffer layer by the suspension plasma spraying using the second suspension.

FIG. 1 is a conceptual diagram for a suspension supplier 1 provided in a suspension plasma spraying apparatus for implementing a method for preparing a two-layer ceramic thermal barrier coating layer in which a buffer layer according to an embodiment of the present disclosure is interposed, in particular, a connected suspension supplier suitable for forming an inclined functional coating layer, such as a two-layer ceramic thermal barrier coating layer in which a buffer layer according to an embodiment of the present disclosure is interposed.

The suspension supplier 1 for the suspension plasma spraying apparatus includes basically a plurality of storage tanks for storing suspensions in which components constituting a raw material of a material constituting a coating layer are dispersed; a transfer pipe for connecting the storage tanks each other and for moving the suspensions or supplying the suspensions to a plasma spraying gun; and an opening/closing valve provided in the transfer pipe for opening and closing a flow of the suspensions.

More specifically, the suspension supplier 1 for the suspension plasma spraying apparatus may include a first suspension storage tank 11 in which the first suspension is stored; a second suspension storage tank 12 in which the second suspension is stored; a first transfer pipe 21 for supplying the first suspension from the first suspension storage tank to a plasma spraying gun; a first opening/closing valve 22 provided in the first transfer pipe for controlling a supply amount of the first suspension to the plasma spraying gun; a second transfer pipe 31 for connecting between the first suspension storage tank and the second suspension storage tank and for transferring the second suspension to the first suspension storage tank; a second opening/closing valve 32 provided in the second transfer pipe for controlling a flow rate of the second suspension to the first suspension storage tank; a third transfer pipe 41 for supplying the second suspension from the second suspension storage tank to a plasma spraying gun; and a third opening/closing valve 42 provided in a third transfer pipe for controlling a supply amount of the second suspension to the plasma spraying gun.

When the method for preparing a ceramic thermal barrier coating layer according to an embodiment of the present disclosure is performed using the suspension supplier 1 for the suspension plasma spraying apparatus as described above, it is possible to more easily realize an inclined functional thermal barrier coating layer, i.e., a two-layer ceramic thermal barrier coating layer having a buffer layer interposed therebetween, and particularly, it is possible to easily obtain a buffer layer in which a content fraction of each component continuously changes from the lowest layer portion to the uppermost layer portion.

According to the method for preparing a two-layer ceramic thermal barrier coating layer having a buffer layer interposed therebetween, using suspension plasma spraying with a connected suspension supplier suitable for forming an inclined functional coating layer as described above, a two-layer ceramic thermal barrier coating layer is formed by suspension plasma spraying with a suspension supplier connected using two types of suspensions of YSZ and rare-earth zirconates containing pulverized oxide particles through high energy milling or alloyed composite oxide particles. Here, the rare-earth zirconates are deposited on an upper layer which is a high-temperature portion of a corresponding coating layer, in which the rare-earth zirconates have a pyrochlore or fluorite or a mixed phase of two phases with low thermal conductivity and excellent phase stability at a high temperature, and there is no m-ZrO₂ causing deterioration of the thermal barrier coating. Here, YSZ is deposited on a lower layer which is a low-temperature portion of the corresponding coating layer, in which the YSZ has a thermal expansion coefficient close to that of a substrate and has excellent high-temperature mechanical properties. Accordingly, it is possible to form a two-layer ceramic thermal barrier coating layer which can be used at higher temperatures as compared with conventional YSZ monolayer coatings, and has improved adhesion and high-temperature durability over rare-earth zirconates monolayer coatings.

Hereinafter, the present disclosure will be described in detail by way of examples to illustrate it. However, the embodiments according to the present disclosure may be modified in various other forms, and the scope of the present specification is not construed as being limited to the above-described embodiments. The embodiments of the present disclosure are provided to more fully describe the present disclosure to a person skilled in the art.

Example 1 to 4

1. Preparation of a Suspension for Suspension Plasma Spraying

Commercial YSZ powder (7.5 wt % Y2O3-ZrO2, PRAXAIR, ZR0271-5, USA, <125 μm) was ball milled for 20 hours using YSZ ball (φ1 mm) and IPA as a mixing medium. Then, it was produced as a suspension by ball milling for 1 hour by dispersing with YSZ ball and ethanol as a mixing medium at a ratio of 1:9 relative to the powder. FIG. 2B is the result of an XRD phase analysis of the commercial YSZ powder used in the suspension of the first suspension storage tank in Examples 1 to 4 of the present disclosure. It was seen that the YSZ powder was composed of a tetragonal prime phase and was pulverized into several μm size powders in the form of several tens μm granules after ball milling for 20 hours.

Further, the powder of La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) were synthesized by using raw material powders of La₂O₃ (High purity chemicals, Japan, 99.99%, 10 μm), Gd₂O₃ (High purity chemicals, Japan, 99.99%, 2-3 μm), and ZrO₂ (High purity chemicals, Japan, 98%, 5 μm) as shown in the composition of Table 1 and produced as a suspension.

TABLE 1 wt % Specimen Composition ZrO₂ La₂O₃ Gd₂O₃ Example 1 La_(1.89)Gd_(0.24)Zr_(2.12)O_(7.06) 45.63 46.92 7.46 Example 2 La_(1.18)Gd_(0.75)Zr_(2.12)O_(7.06) 44.95 33.01 22.04 Example 3 La_(0.71)Gd_(1.38)Zr_(2.12)O_(7.06) 44.29 19.52 36.19 Example 4 La_(0.24)Gd_(1.53)Zr_(2.12)O_(7.06) 43.65 6.41 49.94

Each oxide composition was mixed with a zirconia ball, IPA (Isopropyl alcohol), and 0.5 wt. % dispersant (Dibutyl phosphate, Sigma-Aldrich, USA, 96%) for 24 hours by ball milling. The mixture was heated with stirring using an agitator to evaporate a solvent and then dried in a dryer at 80° C. The dried powder was calcined at 1550° C. for 2 hours and then powder particles were assembled and sieved using induction to prepare a La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) synthetic powder. The prepared La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) powder was dispersed in a ratio of 1:9 with respect to the powder using YSZ ball and ethanol as a mixing medium, and it was produced as a suspension by ball milling for 1 hour. FIG. 2A is the result of an XRD phase analysis of La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) powder after the synthesis used in the suspension of the second suspension storage tank in Examples 1 to 4 of the present disclosure. It was seen that the synthesized La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06)=0.24, 0.71, 1.18, 1.65) powder has a pyrochlore crystal phase clearly showing superlattice peaks of (331) and (511), and also has a fluorite phase.

2. Formation of a Thermal Barrier Coating Layer Using Suspension Plasma Spraying

The prepared slurry was deposited using suspension plasma spraying (Axial III plasma spraying system, Northwest Mettech Corp., Canada) on a substrate that Amdry 386-2 (Sulzer metco, Switzerland) Bond coat composition was deposited on a nickel-based superalloy substrate to a thickness of about 200 μm by high-velocity oxy-fuel spraying (HVOF). Coating was carried out with the following coating conditions: a mixing ratio of Ar, H₂, and N₂ was controlled to 7.5:1.5:1, a distance between a coated substrate and a plasma torch was 75 mm, a rotation speed of the coated substrate was 1500 rpm, a suspension supply rate was 45 mL/min, and coating pressurized voltage and current were 150V and 220 A. FIG. 1 shows a schematic view of a slurry supply apparatus for preparing a two-layer ceramic thermal spray coating by a suspension plasma spraying technique. First, an amount of slurry consumption over time for a slurry fed at a feeding rate of 45 ml per minute was calculated. Then, a two-layer ceramic thermal barrier coating was prepared in such a manner that a slurry was additionally supplied to the first suspension storage tank from the second suspension storage tank immediately before the completion of slurry supply from the first suspension storage tank.

In the case of a coating specimen of La_(1.66)Gd_(0.24)Zr_(2.12)O_(7.06)/YSZ in Example 1, first, a suspension of a YSZ composition in the first suspension storage tank was supplied to plasma flame for deposition for 40 minutes. When the supply of the suspension of the YSZ composition was carried out for 30 minutes, a slurry having a composition of La_(1.66)Gd_(0.24)Zr_(2.12)O_(7.06) was poured into the first suspension storage tank from the second suspension storage tank, and a YSZ+ La_(1.66)Gd_(0.24)Zr_(2.12)O_(7.06) layer was coated for 10 minutes. Also, coating was performed so that a composition of La₂Zr₂O₇ in the second suspension storage tank was further supplied for 30 minutes, and deposition was performed for 1 hour and 10 minutes in total.

Comparative Example

Formation of a Thermal Barrier Coating Layer Using Atmospheric Plasma Spraying

Commercial YSZ powder (7.5 wt % Y₂O₃—ZrO₂, PRAXAIR, ZR0271-5, USA, <125 μm) was deposited using atmospheric plasma spraying (TripleX Pro, Oerikon Metco, Switzerland) on a substrate that Amdry 386-2 (Sulzer metco, Switzerland) Bond coat composition was deposited on a nickel-based superalloy substrate to a thickness of about 200 μm by high-velocity oxy-fuel spraying (HVOF).

The coating was carried out with the following coating conditions: a gun speed was controlled at 1000 mm/sec, a distance between a coated substrate and a plasma torch was 125 mm, a powder feed rate was 10 g/min, and coating pressure voltage and current were 102 V and 500 A.

<Experimental Example 1> Crystal Structure Analysis and Cross-Sectional Microstructure Observation for Specimens Prepared in Examples and Comparative Example

FIG. 3 is the result of X-ray diffraction (XRD) analysis for the specimen of the two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and the specimen of the monolayer thermal barrier coating prepared in Comparative Example.

FIGS. 4A and 4B are images of a scanning electron microscope (SEM) showing a cross-sectional microstructure for the specimen of the two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and the specimen of the monolayer thermal barrier coating prepared in Comparative Example.

In accordance with FIG. 3, it could be seen that it consists of a single phase of fluorite which appears from the upper layer, i.e., the La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x=0.24, 0.71, 1.18, 1.65) layer. In addition to a peak from the fluorite phase, a peak of the t′-YSZ located in the lower was also weakly observed. In accordance with the cross-sectional microstructure in FIG. 4A, a ceramic coating layer was deposited to a thickness of approximately 480˜550 μm. A thickness ratio of the YSZ layer as the lower layer and the La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) layer as the upper layer was almost the same as 1:1, but the total thickness was different. The La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) layer and the YSZ layer each had a dense microstructure and showed a vertical separation microstructure such as a slight vertical crack.

On the other hand, in accordance with FIGS. 3 and 4B, in the case of the specimen of the monolayer coating layer having the commercial YSZ composition and prepared using atmospheric plasma spraying in Comparative Example, it could be seen that a coating having a pore similar in size to a granule size composed of a single phase of the t′-YSZ was formed. Microstructural observation revealed a vertical separation microstructure such as a slight vertical crack.

FIG. 5 is an image of a scanning electron microscope (SEM) showing a cross-sectional microstructure for a specimen of a two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and the result of EDS line scanning showing the distribution of La, Gd, Zr, and Y elements. It could be seen that a La/Gd layer, a Zr layer, and a Y layer are clearly distinguished by the difference in the composition of the lower layer, i.e., the YSZ layer and the upper layer, i.e., the La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) layer. From this, a thickness of the upper layer, an intermediate layer, and the lower layer, which were not clearly distinguished from the SEM cross-sectional microstructure, were more clearly known. It was also seen that a thickness of the intermediate layer was 150˜200 μm.

<Experimental Example 2> Measurement for Adhesion and Thermal Durability on Specimens Prepared in Examples and Comparative Example

TABLE 2 Specimen Adhesion (MPa) Example 1 >29.9 Example 2 26.8 Example 3 23.7 Example 4 >24.1 Comparative Example >28.1

FIG. 7 is the result of an isothermal deterioration test at 1275° C. for the specimen of the two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 and the specimen of the monolayer YSZ thermal barrier coating prepared in Comparative Example, and it shows the number of cycles until a coating is soundly attached to a substrate. It was seen that the specimen of the two-layer ceramic thermal barrier coating prepared in Examples 1 to 4 has higher thermal durability compared to the specimen of the monolayer YSZ thermal barrier coating prepared in Comparative Example through the 1275° C. isothermal degradation test. Considering that a temperature range where the t′-YSZ stably exists is from room temperature to 1200° C., it is believed that the two-layer ceramic thermal barrier coating having stable rare-earth zirconates as an upper layer in experimental conditions of a high-temperature of 1275° C. provides more improved high-temperature durability. 

1. A method for preparing a ceramic thermal barrier coating layer comprising: (a) preparing a first suspension in which first oxide particles for thermal barrier coating are dispersed and a second suspension in which second oxide particles for thermal barrier coating are dispersed, respectively; (b) forming a first coating layer on a base material by suspension plasma spraying (SPS) using the first suspension; (c) forming a buffer layer on the first coating layer by the suspension plasma spraying using a mixed suspension of the first suspension and the second suspension; and (d) forming a second coating layer on the buffer layer by the suspension plasma spraying using the second suspension.
 2. The method of claim 1, wherein a first oxide for thermal barrier coating is YSZ (Yttria-stabilized zirconia) and a second oxide for thermal barrier coating is rare-earth zirconates.
 3. The method of claim 2, wherein the rare-earth zirconates are La_(1.89−x)Gd_(x)Zr_(2.12)O_(7.06) (x is 0.24 to 1.65).
 4. The method of claim 1, wherein the step (a) comprises: (a-1) mechanically pulverizing an oxide powder for thermal barrier coating; and (a-2) forming a slurry containing the pulverized oxide powder.
 5. The method of claim 1, wherein the step (a) comprises: (a-1) mixing two or more oxide powders to perform mechanical alloying; and (a-2) forming a slurry containing an oxide powder for thermal spraying coating obtained in the step (a-1).
 6. The method of claim 5, wherein the two or more oxide powders comprise (i) one or more selected from Y₂O₃, Gd₂O₃, and La₂O₃, and (ii) one or more selected from ZrO₂ and CeO₂.
 7. The method of claim 4, wherein the mechanical pulverizing or mechanical alloying of the oxide powder in the step (a-1) is performed by planetary ball milling, attrition milling, or shaker milling.
 8. The method of claim 4, further comprising, prior to the step (a-2), calcining the oxide powder obtained in the step (a-1).
 9. The method of claim 1, wherein, in the step (c), the buffer layer is formed so that a content ratio of the first oxide and the second oxide continuously changes in a thickness direction of a coating layer.
 10. The method of claim 9, wherein the step (c) is carried out using a suspension plasma spraying apparatus with a suspension supplier, wherein the suspension supplier comprises: a first suspension storage tank in which the first suspension is stored; a second suspension storage tank in which the second suspension is stored; a first transfer pipe for supplying the first suspension from the first suspension storage tank to a plasma spraying gun; a first opening/closing valve provided in the first transfer pipe for controlling a supply amount of the first suspension to the plasma spraying gun; a second transfer pipe for connecting between the first suspension storage tank and the second suspension storage tank and for transferring the second suspension to the first suspension storage tank; a second opening/closing valve provided in the second transfer pipe for controlling a flow rate of the second suspension to the first suspension storage tank; a third transfer pipe for supplying the second suspension from the second suspension storage tank to the plasma spraying gun; and a third opening/closing valve provided in the third transfer pipe for controlling a supply amount of the second suspension to the plasma spraying gun.
 11. The method of claim 5, wherein the mechanical pulverizing or mechanical alloying of the oxide powder in the step (a-1) is performed by planetary ball milling, attrition milling, or shaker milling.
 12. The method of claim 5, further comprising, prior to the step (a-2), calcining the oxide powder obtained in the step (a-1). 